1. Field of the Invention
The present invention relates to a radiation imaging system and an operating method thereof, and a radiation image detecting device.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using X-rays, as a kind of radiation, is known. The X-ray imaging system is constituted of an X-ray generating apparatus for generating the X-rays and an X-ray imaging apparatus, which receives the X-rays and takes an X-ray image. The X-ray generating apparatus includes an X-ray source for emitting the X-rays to an object, a source control device for controlling the operation of the X-ray source, and an emission switch for inputting an emission start command of the X-rays. The X-ray imaging apparatus includes an X-ray image detecting device, which detects the X-ray image upon receiving the X-rays passed through the object, and a console, which controls the operation of the X-ray image detecting device and applies various types of image processes to the X-ray image.
Recently, in a field of the X-ray imaging system, an X-ray image detecting device that uses a flat panel detector (FPD) as a detection panel, instead of an X-ray film or an imaging plate (IP), becomes widespread. The FPD has a matrix of pixels each for accumulating signal charge in accordance with the amount of X-rays incident thereon. The FPD accumulates the signal charge on a pixel-by-pixel basis. The FPD converts the accumulated signal charge into a voltage signal at its signal processing circuit, and thereby detects the X-ray image representing image information of the object and outputs the X-ray image as digital image data.
An electronic cassette (portable X-ray image detecting device) that is composed of the FPD contained in a rectangular parallelepiped housing is in practical use. The electronic cassette is used while being loaded detachably into an existing imaging stand sharable with a film cassette and an IP cassette or a specific imaging stand designed for the electronic cassette, in contrast to a non-detachable type. Furthermore, the electronic cassette is used while being put on a bed or held by the object himself/herself, to take an image of a body part that is hard to take with the non-detachable type. The electronic cassette is sometimes brought out from a hospital to a place having no imaging stand, for use in bedside radiography of an elder patient or in urgent radiography of an injured patient, natural disaster victims, or the like.
Also, for the purpose of obtaining a radiographic image of appropriate image quality with reducing radiation exposure of the object, the X-ray imaging system sometimes performs automatic exposure control (AEC) in which an integrated value (accumulative dose) of an X-ray dose is measured during exposure to (irradiation with) the X-rays and X-ray emission from the X-ray source is stopped as soon as the accumulative dose has reached a target value. The X-ray dose applied by the X-ray source is determined by a tube current-emission time product (mAs value), being a product of an emission time (in seconds) of the X-rays and a tube current (in mA) for defining the X-ray dose applied per unit of time by the X-ray source. Although imaging conditions, including the emission time and the tube current, have approximate recommended values in accordance with a body part (chest or head) to be imaged, sex, age, and the like of the object, X-ray transmittance varies depending on the individual difference such as physique of the object. Therefore, the automatic exposure control is carried out in order to obtain more appropriate image quality.
In carrying out the automatic exposure control, the X-ray imaging system is provided with a dose detection sensor for detecting the X-ray dose passed through the object. If there are provided a plurality of dose detection sensors, a measurement area (radiation dose measurement area) that corresponds to an area (also called a region of interest or a ROI) that is the most noteworthy in making a diagnosis is set in accordance with a body part to be imaged. The timing of stopping the X-ray emission is judged based on the X-ray dose detected by the dose detection sensor disposed in the measurement area.
According to U.S. Pat. No. 7,734,013, the dose detection sensor that is disposed in a position corresponding to a mammary gland, being the measurement area, is extracted based on output of the plurality of dose detection sensors (AEC sensors) in pre-exposure, to use the extracted dose detection sensor as a sensor in actual exposure. The position of the mammary gland is determined using an average of output of the dose detection sensors.
European Patent Publication No. 1001665 A2 discloses a radiation imaging system that uses signal lines (data lines) of pixels in the AEC. The signal line of the pixels corresponding to the measurement area is automatically chosen with a program.
According to Japanese Patent Application Publication No. 09-055298, a non-irradiation field, a directly exposed area (directly irradiation field), and the measurement area (irradiation field of the object) are recognized by a histogram analysis of pixel values of a radiographic image. A dose detection sensor (imaging sensor) corresponding to the measurement area is chosen by comparison with a typical value of a pixel in the measurement area.
According to Japanese Patent Application Publication No. 2005-476802, in a radiation image reading device using a CMOS image sensor, a stop of the X-ray emission is judged based on a signal of a pixel having the highest signal value (in the directly exposed area), out of all pixels of the CMOS image sensor.
In setting the measurement area, requiring the pre-exposure as described in the U.S. Pat. No. 7,734,013 or the histogram analysis as described in the Japanese Patent Application Publication No. 09-055298 causes complication of a process and increase in time of the process. Also there is a problem that the pre-exposure or radiography poses unnecessary exposure of the patient.
In the European Patent Publication No. 1001665 A2, the data line of the pixels corresponding to the measurement area is automatically chosen, but it does not describe how to choose the data line. The Japanese Patent Application Publication No. 2005-143802 is aims at judging an end of the X-ray emission, and is unsuitable for the AEC for judging when to stop the X-ray emission.